PPP is a semiconductive polymer having low conductivity. Its general formula is (C.sub.6 H.sub.4).sub.x, where x is the degree of polymerization. PPP becomes electrically conductive only when doped by electron acceptors (P type doping) or by electron donors (N type doping).
In the above-mentioned applications, doped or undoped PPP is advantageously used in the form of a very thin film. In particular:
the photovoltaic application requires thicknesses of less than 10.sup.-7 m in order to minimize resistance; and
the electrochromic application requires thicknesses of less than 1 micron in order to retain an adequate switching speed.
No prior art method of preparing PPP is capable of providing it in the form of a thin film having uniform and well-determined characteristics.
Thus, for example, the methods of synthesis described by Kovacic et al (P. Kovacic, A. Kyriakis, J.Am. Chem. Soc. 85, 454 (1963)), or by Fauvarque et al (J. F. Fauvarque, M. A. Petit, F. Pfluger, A. Jutand, C. Chevrot, M. Troupel, Makromol Chem. Rapid Commun. 4, 455 (1983)), lead to the formation of powders which are not suitable for providing films since they are insoluble and unmeltable. A pelleting press cannot be used to obtain thicknesses of less than 30 .mu.m (because of mechanical strength problems). It is thus necessary and advantageous to synthesize PPP directly in the form of thin films.
Other writers have proposed obtaining films by electrosynthesis by oxidizing benzene under the following conditions:
oxidizing benzene or biphenyl in acetonitrile (T. Osa, A. Yildiz, T. Kurvana--J.A.C.S. 91 (14) (1969) 3994); PA1 oxidizing benzene in SO.sub.2 at -20.degree. C. (M. Delamar, P. C. Lacaze, J. Y. Dumousseau, J. E. Dubois--Electrochimica acta 27-1 (1982) 61); PA1 oxidizing benzene at the benzene-HF interface (A. F. Shepard, B. F. Dannels--J polym. sci. A.1 4 (1966) 511, and I. Rubinstein--electrochem. soc. 130.7 (1983) 1506); and PA1 oxidizing benzene in nitrobenzene (K. Kaeriyanma, M. A. Sato, K. Someno, S. Tanaka--J. Chem. soc. chem. commun. (1984) 1199). PA1 preparation is difficult (in HF or liquid SO.sub.2), or takes place under limiting conditions of solvent use; PA1 there is no guarantee that the phenylene units will be chained in the para positions only; and
However, these four methods provide films which suffer from the following defects:
OH, oxygen or nitrogen are present in the film.
Other writers have proposed obtaining films by electrosynthesis by reducing the following in acetonitrile: ##STR2## (G. Schiavon, G. Zotti, G. Bontempelli--J. electro. anal. chem. 161 (1984) 323).
This method gives a film in which the phenyl units are chained in the para positions, but in which a nickel atom is incorporated once every 7 or 8 cycles.